The present invention is related to memory devices for electronic applications. More particularly, the present invention provides systems and methods for enabling backward compatibility for new memory devices with relaxed or less stringent performance parameters with legacy systems that expect stringent adherence to legacy performance parameters. The enabling of backward compatibility means that the new memory devices may be used in legacy systems. The present invention in enabling backwards compatibility may be applied to memory devices, such as DRAM (Dynamic Random Access Memory) devices, which can be applied to memory systems, modules, cells, and the like.
In memory systems, two general classes of memories exist. Such classes include low latency memories. The low latency memories have effectively infinite endurance or usage-cycles and do not degrade with respect to age or repeated accesses. Additionally, such classes also include relatively longer latency memories that do not have infinite endurance or usage cycles, and may degrade with respect to age or repeated accesses. In the case of the relatively long latency memories, sophisticated multi-error detection and correction algorithms have been implemented to correct for data cells that can degrade over the lifetime of the device due to aging effects or repeated accesses. In the case of low latency memories such as DRAM devices, however, effectively infinite endurance or usage-cycles are assumed so once weak bits or bad bits are mapped out by the device manufacturer, no errors should occur due to degradation of data cells due to aging effects or repeated accesses.
As is known, the conventional DRAM memory cell has an access transistor and a storage capacitor. The access transistor connects with the storage capacitor to a bitline when switched-on such that the capacitor stores the logic value placed on the bitline. Due to the tendency of a capacitor to lose its charge over time, DRAM memory cells must be periodically ‘refreshed’, which serves to maintain the value stored in each storage capacitor at its desired value. The amount of time that a cell can retain its logic value is referred to as its “data retention time”.
A trend in the development of memory cells is that the cells have been shrinking due to advancements in process technology and the demand for ever larger memory capacity. This necessarily results in a reduction in the sizes of the access transistor and storage capacitor, which can lead to several limitations. For example, each access transistor exhibits leakage which acts to slowly drain stored charge from the storage capacitor. This leakage characteristic—and thus each cell's data retention time—varies from transistor to transistor; however, this variability increases as the size of the access transistors is reduced. Another problem is that a shrinking memory cell results in a smaller storage capacitor, and thus a reduced storage capacitance. This can also adversely affect the data retention time characteristics of the cells.
Although highly successful, low-latency memories have limitations. Thus, improvements in memory cell testing and manufacturing are highly desired.